1. Field of the Disclosure
The disclosure is related generally to measuring induction in a borehole and provides an apparatus and methods for reducing the effects of borehole currents in induction measurements.
2. Background of the Art
Electromagnetic induction resistivity well logging instruments are well known in the art and are used to determine the electrical conductivity, and its converse, resistivity, of earth formations penetrated by a borehole. In electromagnetic induction resistivity well logging, eddy currents are induced in a region of the formation adjoining the borehole. These eddy currents give rise to magnetic fields having properties that are related to the region of the formation. Formation conductivity may be determined by measuring these magnetic fields. The formation conductivity is used for, among other things, inferring a fluid content of the earth formations. For example, low conductivity (high resistivity) is typically associated with hydrocarbon-bearing earth formations. The physical principles of electromagnetic induction well logging are well described, for example, in J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists, December 1962. Many improvements and modifications to the electromagnetic induction resistivity instruments described in the Moran and Kunz reference have been made. In an exemplary modification, a multi-component logging tool includes transmitters and receivers substantially oriented along axes of an orthogonal coordinate system associated with the logging tool.
When logging an earth formation, various non-formation effects may arise which may degrade the accuracy of the log. Typical non-formation effects include signals induced in a borehole mud, signals due to eccentricity of the logging tool within the borehole, signals resulting from misalignment of transmitters and or receivers with respect to the tool axis, among others. Tool eccentricity within the borehole generally causes more non-formation effects in transverse (X-oriented or Y-oriented) transmitters and receivers than in longitudinal (Z-oriented) transmitters and receivers. These borehole eccentricity effects are generally at a minimum when the magnetic fields produced by the currents circulating in the mud near the receivers cancel each other. This cancellation often takes place when the logging tool is positioned at the center of the borehole. However, maintaining this central position in horizontal boreholes or in highly-deviated boreholes is either difficult or unattainable using present logging tool designs. A deviated borehole is one in which the axis of the borehole is inclined to the vertical, Thus, there is a need for a logging tool that reduces borehole effects in multi-component resistivity measurements.